The present invention relates to a shrinkable polyester film, and more in detail, relates to a shrinkable polyester film suitable for labels used on PET (polyethylene terephthalate) bottles or other bottles and for various packaging.
Hitherto, as the shrinkable film used for packaging or labelling, a film of polyvinyl chloride, a film of polystyrene, etc. have been broadly used. However, the polyvinyl chloride film has the difficulties in disposal, that is, the polyvinyl chloride film has a high combustion calorie and generates harmful hydrogen chloride when it is burnt. And the polystyrene film has a defect of poor resistance to weather. Accordingly, a film of the other polymer which is suitable for the purpose has been desired.
Hereupon, as the characteristics necessary for the shrinkable film for packaging and labelling, the sufficient shrinkage, the resistance to solvents such as printing ink, the shrinking property not to cause wrinkles on the label at the time of shrinking treatment, the warm water-resistance not to cause wrinkles, slack and whitening, etc. in the warm water treatment carried out after shrinking treatment of the film for the sterilization of the bottle may be mentioned.
For the above-mentioned demands, it has been disclosed that a film having a sufficient shrinkage and being excellent in heat sealing property can be obtained by monoaxially stretching a noncrystalline copolyester, a blend of a noncrystalline copolyester and a crystalline polyester or a blend of two or more kinds of crystalline polyester in the machine direction or the transverse direction (refer to Japanese Patent Applications Laid-Open (KOKAI) Nos. 57-42726(1982), 57-159618(1982) and 59-97175(1984)). However, even in such a film, the satisfactory shrinking properties and the warm water-resistance could not be obtained, and particularly concerning the shrinkable properties, such a film is apt to cause wrinkles at the time of shrinking treatment as compared to the conventional films of polyvinyl chloride, etc. Namely, the above films hitherto proposed encounter with a severe problem in the practical use.
Furthermore, in recent years, it has been desired to use a shrinkable film to the containers of various shapes, and the respectively different characteristics have been demanded corresponding to the properties of the various containers, particularly widely used containers for the soft drinks such as PET bottle, glass bottles of a large type with a small mouth, heat-resistant PET bottle and non heat-resistant PET bottle. Accordingly, it has been demanded to obtain a shrinkable film of polyesters which satisfies the respective characteristics.